Read Maty Ezraty's repsonse:
Building strength in the ankles is one of the benefits of practicing the four Prasarita Padottanasana postures consecutively. The benefits of a pose, however, are often the risks of that pose. Therefore, it is important to build strength gradually and avoid creating unnecessary pain or causing injury. These four linked poses are a challenge. Many beginning students have trouble completing them. This is not an uncommon complaint with new students and not necessarily a specific challenge to those who are overweight. And yes, I think she can overcome this with time and practice.
My first suggestion is that you consider breaking up the Parasarita Padottanasanas by having her step back into Tadasana/Samasthiti (Mountain Pose/Equal Standing Pose) after each variation. This way, your student is still working on all four of the variations, but she has time to recover in between. When she has developed more strength, then you can have her link the first two variations, step to Tadasana, and then step into the second two. This would be a more gradual way to build up the strength in her ankles. Vinyasa means gradual progression or progress, and it needs to suit the individual.
Your intuition is correct to watch to see that the four corners of her feet are properly pressed into the ground. I would further suggest that you look carefully to see if her outer ankles are puffing or her inner ankles are falling. She may be pushing her outer ankles out too much. This can happen when a student misunderstands the instruction of grounding the outer edges of the feet. Instead, the student overdoes this instruction and overextends the work of the outer ankles, causing discomfort. This is common with students who have high arches or are trying to lift the arches of their feet incorrectly. It is important to evenly ground the inner heel, the mount of the big toe, and the outer edge of the foot. Then we must learn to lift up out of our ankles properly. The inner and outer ankles should draw in and up evenly. The outer ankles should be shapely, not puffy.
Finally, check that your student is working the legs strongly. The four corners of her knees should be strongly lifting upward off her ankles. The four sides of the thighs need to lift upward. Correct work in the legs could ease the work, and the load, of the ankles and feet. A pain in the body is often caused by what is above. In this case, what is above may be lazy, which is why I am asking you to check, and perhaps correct, the work of the knees and thighs.
Maty Ezraty is co-creator of the first two Yoga Works yoga studios in Santa Monica, California. A former YJ Asana columnist, she travels around the world leading teacher trainings, workshops, and yoga retreats.